Method of identifying defect distribution in silicon single crystal ingot

ABSTRACT

A surface of a reference sample is contaminated with a transition metal, and a heat treatment is performed to diffuse the transition metal in the sample. A concentration of recombination centers formed by the transition metal is measured in the entire heat-treated reference sample, and a region [V], a region [Pv], a region [Pi], and a region [I] in the reference sample are defined based on the values measured. Meanwhile, recombination lifetimes associated with the transition metal are measured in the entire heat-treated reference sample. Based on both of the measurement results, a correlation line of the concentration of recombination centers and the recombination lifetimes is produced. A surface of the measurement sample is contaminated with the transition metal, and a heat treatment is performed to diffuse the transition metal in the sample. Recombination lifetimes associated with the transition metal are measured in the entire heat-treated measurement sample, and the values measured are checked against the correlation line to infer the region [Pv] and the region [Pi] as well as the boundary thereof in the measurement sample.

TECHNICAL FIELD

The present invention relates to a method of identifying defectdistribution in a silicon single crystal ingot (hereinafter referred toas an “ingot”) pulled according to a Czochralski method (hereinafterreferred to as a “CZ method”). Particularly, the invention relates to amethod of identifying a grown-in defect free region in an ingot in whichthe concentration of oxygen dissolved in the ingot is 1.2×10¹⁸ atoms/cm³(old ASTM, likewise hereinafter) or higher, or 9.0×10¹⁷ atoms/cm³ orlower, and to a method of identifying a grown-in defect-free region in alow-oxygen concentration ingot in which the concentration of oxygendissolved in the ingot is within the range of 8.0×10¹⁷ to 1.0×10¹⁸atoms/cm³.

BACKGROUND OF THE INVENTION

With the recent trend of super-miniaturization in semiconductorintegrated circuits, it has been suggested that reduction in deviceyield arises from the presence of crystal originated particle(hereinafter referred to as “COP”), microdefects of oxygen precipitatesthat become the nuclei of oxidation induced stacking fault (hereinafterreferred to as “OISF”), interstitial-type large dislocation (hereinafterreferred to as “L/D”), and the like.

COP is a pit originated from a crystal that appears on a wafer surfacewhen the silicon wafer that has undergone a mirror polishing issubjected a SC-1 rinse with the use of a mixed solution of ammonia andhydrogen peroxide. When the wafer is measured with a particle counter,the pit is detected as a particle (light point defect, LPD). The COPbecomes a cause of deteriorating electrical characteristics such as timedependent dielectric breakdown (TDDB) characteristic and time zerodielectric breakdown (TZDB) characteristic of oxide films. In addition,COP existing on a wafer surface can create a height difference in awiring process of devices, which can become a cause of wire breakage. Inaddition, it becomes a cause of leakage or the like in theelement-isolating portions, lowering product yield.

It is considered that OISF originates from micro oxygen precipitatesformed during crystal growth, which form the nuclei thereof; and it is astacking fault that is exposed during a thermal oxidation process or thelike in the manufacture of semiconductor devices. This OISF becomes acause of faults in devices, such as an increase in leakage current. L/Dis also called dislocation cluster, or dislocation pit, because asilicon wafer having this defect forms an etching pit having anorientation when it is immersed in a selective etchant solutionincluding hydrofluoric acid as a main component. This L/D also becomes acause of deteriorating electrical characteristics, such as leakagecharacteristics, isolation characteristics, and the like.

For the reasons stated above, it has been necessary to reduce COP, OISFand L/D defects in silicon wafers used in the manufacture ofsemiconductor integrated circuits.

U.S. Pat. No. 6,045,610 and the corresponding Japanese Unexamined PatentPublication No. 11-1393 discloses a defect-free ingot that does not havethese COP, OISF, and L/D, and a silicon wafer sliced from the ingot.This defect-free ingot is an ingot composed of a perfect region [P],where [P] is a perfect region in which neither agglomerates ofvacancy-type point defects nor agglomerates of interstitial silicon-typepoint defects are detected in an ingot. The perfect region [P] existsbetween a region [V] and a region [I] in an ingot; in the region [V],vacancy-type point defects are predominant and defects in whichsupersaturated vacancies are agglomerated are contained, whereas in theregion [I], interstitial silicon-type point defects are predominant anddefects in which supersaturated interstitial silicons are agglomeratedare contained.

Japanese Unexamined Patent Publication No. 2001-102385 shows that theperfect region [P], which does not have defects in which point defectsare agglomerated, is classified into a region [Pv] in which vacancy-typepoint defects are predominant and a region [Pi] in which interstitialsilicon-type point defects are predominant. The region [Pv] is a regionthat is adjacent to the region [V] and has a concentration ofvacancy-type point defects that is less than the minimum concentrationof vacancy-type point defects at which an OISF nucleus can be formed.The region [Pi] is a region that is adjacent to the region [I] and has aconcentration of interstitial silicon-type point defects less than theminimum concentration of interstitial silicon-type point defects atwhich an interstitial-type large dislocation can be formed.

The ingot composed of the perfect region [P] is produced within therange of a V/G ratio (mm²/minute·° C.) such that OISF (P band) generatedin a ring-like shape during a thermal oxidation treatment disappearsfrom the central area of the wafer and that L/D (B band) does not occur,where the pull rate of the ingot is V (mm/minute) and the temperaturegradient with respect to the ingot's axial direction in the vicinity ofthe solid-liquid interface between the silicon melt and the siliconingot is G (° C./mm).

Conventionally, the following method has been adopted in order tomeasure a distribution of secondary defects generated in an ingot by aheat treatment, that is, agglomerated defects, over the axial directionand over the diametric directions of the ingot. First, an ingot issliced in the axial direction to prepare samples. Then, these samplesare subjected to a mirror etching, are then heat-treated at 800° C. for4 hours in a nitrogen or oxidizing atmosphere, and are subsequentlyfurther heat-treated at 1000° C. for 16 hours. The heat-treated samplesare measured with the use of such methods as copper decoration,Secco-etching, X-ray topography analysis, and lifetime measurement.Generally, the density of the oxygen precipitates formed in a sample bya heat treatment is substantially proportionate to the concentration ofoxygen. When the concentration of oxygen dissolved in an ingot is lessthan 1.2×10¹⁸ atoms/cm³ and more than 9.0×10¹⁷ atoms/cm³, oxygenprecipitates appear at a high density in the ingot due to the heattreatment and therefore the foregoing methods are capable of clearlydistinguishing a region [V], a P band, a region [Pv], a region [Pi], a Bband, and a region [I].

However, when the concentration of oxygen dissolved in an ingot is lowand less than 1.0×10¹⁷ atoms/cm³, such as in the case of the ingot thatis pulled with a magnetic field being applied according to an MCZ(magnetic-field-applied CZ) method, the density of the oxygenprecipitates that occur due to the heat treatment is not sufficientlyhigh. For this reason, the above-described conventional method suffersfrom a problem in that those regions cannot be distinguished clearly.For example, when recombination lifetimes are measured subsequent to aheat treatment, the difference between the measurement value of therecombination lifetime in the region [Pv] and the measurement value ofthe recombination lifetime in the region [Pi] becomes smaller in asample having a low oxygen concentration than in a sample having highoxygen concentration. For this reason, there has been a drawback in thatthe boundary between the region [Pv] and the region [Pi] cannot bedistinguished clearly with samples having low oxygen concentrations.Moreover, depending on conditions of the heat treatment, the boundarybetween the region [Pv] and the region [Pi] shifts toward either theregion [Pv] side or the region [Pi] side. This is attributed to the factthat the density and size of the oxygen precipitates change in thesample depending on heat treatment conditions. For this reason, it hasbeen difficult to measure real point defect regions at high precision.In addition, when the concentration of oxygen dissolved in the ingot isnot more than 9.0×10¹⁷ atoms/cm³, it has been impossible to clearlydistinguish the boundary between the region [Pv] and the region [Pi],and moreover, it has been difficult to clearly measure the P band, whichcorresponds to the boundary between the region [V] and the region [Pv],and the B band, which corresponds to the boundary between the region[Pi] and the region [I].

Furthermore, when the concentration of oxygen dissolved in the ingot ishigh, 1.2×10¹⁸ atoms/cm³ or higher, the density of the oxygenprecipitates that appear due to the heat treatment is too high;therefore, with the above-described conventional methods, it has beendifficult to precisely measure the P band, which corresponds to theboundary between the region [V] and the region [Pv].

It is a first object of the present invention to provide a method ofeasily identifying the region [Pv] and the region [Pi] in an ingot aswell as the boundaries thereof even when the concentration of oxygencontained in the ingot is low.

It is a second object of the present invention to provide a method ofidentifying a defect distribution in a silicon single crystal ingot, bywhich the boundary between the region [Pv] and the region [V] in aningot can be easily identified even when the concentration of oxygendissolved in the ingot is high.

It is a third object of the present invention to provide a method ofidentifying defect distribution in a silicon single crystal ingot thateasily identifies both the boundary between the region [V] and theregion [Pv] and the boundary between the region [Pi] and the region [I]in the ingot even when the concentration of oxygen dissolved in theingot is low.

SUMMARY OF THE INVENTION

A first aspect of the present invention pertains first and thirdidentification methods, and it is a method of identifying a point defectdistribution in a silicon single crystal ingot, including the steps of:(a) slicing a first silicon single crystal ingot in an axial directionthereof, the ingot pulled from a first silicon melt at a varying pullrate, to prepare a reference sample including a region [V], a region[Pv], a region [Pi], and a region [I]; (b) coating a surface of thereference sample with a transition metal solution in which a transitionmetal is dissolved at a concentration of 1 to 1000 ppm tometal-contaminate the reference sample; (c) heat-treating themetal-contaminated reference sample in an atmosphere of argon, nitrogen,oxygen, hydrogen, or a mixed gas thereof either at temperatures of 600to 1200° C. for 0.5 to 24 hours while increasing the temperature at arate of 0.5 to 10° C./minute, or at temperatures of 600 to 1100° C. for10 to 60 seconds while increasing the temperature at a rate of 30 to 70°C./second, to diffuse the transition metal in the reference sample; (d)measuring a concentration of recombination centers formed by thetransition metal in the entire heat-treated reference sample; (e)measuring recombination lifetimes associated with the transition metalin the entire heat-treated reference sample; (f) producing a correlationline between the concentration of recombination centers and therecombination lifetimes from measurement results obtained in the step(d) and the step (e), and defining regions including at least the region[Pv] and the region [Pi] as well as a boundary thereof in the referencesample; (g) slicing a second silicon single crystal ingot, in an axialdirection thereof, the second silicon single crystal ingot pulled from asecond silicon melt at a predetermined pull rate, to prepare ameasurement sample including at least a region [Pv] and a region [Pi];(h) coating a surface of the measurement sample with the same transitionmetal solution as the transition metal solution to metal-contaminate themeasurement sample; (i) heat-treating the metal-contaminated measurementsample under the same conditions as those in the step (c) to diffuse thetransition metal in the measurement sample; (j) measuring arecombination lifetime associated with the transition metal in theentire heat-treated measurement sample; and (k) checking results of themeasuring in the step (j) against the correlation line to infer theregion [Pv] and the region [Pi] as well as a boundary thereof.

The above-described reference sample and measurement sample that havebeen sliced are such that an oxygen concentration thereof is within therange of 8.0×10¹⁷ to 1.0×10¹⁸ atoms/cm³, or that a boundary between theregion [Pv] and the region [Pi] is unidentifiable in the samples whentheir recombination lifetimes are measured after the samples areheat-treated at 800° C. for 4 hours in a nitrogen atmosphere andsubsequently further heat-treated at 1000° C. for 16 hours. The region[V] is a region in which vacancy-type point defects are predominant anddefects in which excessive vacancies are agglomerated are contained, theregion [Pv] is a region in which vacancy-type point defects arepredominant and defects in which vacancies are agglomerated are notcontained, the region [Pi] is a region in which interstitialsilicon-type point defects are predominant and defects in whichinterstitial silicons are agglomerated are not contained, and the region[I] is a region in which interstitial silicon-type point defects arepredominant and defects in which interstitial silicons are agglomeratedare contained.

The first aspect according to the invention has been accomplished basedon the following findings. Although easy and convenient, the methods ofmeasuring recombination lifetime that are represented by an LM-PCD(laser/microwave photoconductivity decay) method cannot identify theregion [Pv] and the region [Pi] as well as the boundary thereof in asample having a low oxygen concentration that have undergone apredetermined heat treatment. On the other hand, the methods ofmeasuring a concentration of recombination centers that are representedby a DLTS (deep level transient spectroscopy) method can identify theregion [Pv] and the region [Pi] as well as the boundary thereof in asample having a low oxygen concentration that has undergone apredetermined heat treatment, although they require many process stepsand much time for the measurement. The present inventors have takennotice that there is a correlation between measurement values that areobtained by the DLTS method for a reference sample pulled at a varyingrate and measurement values that are obtained by the LM-PCD method forthe reference sample. Specifically, the boundaries between respectiveregions are identified in a reference sample using the DLTS method, anda calibration line is produced from the correlation. For a measurementsample pulled at a predetermined rate, the recombination lifetime ismeasured using the LM-PCD method that is easy-to-use, and the measuredvalues are applied to the calibration line to obtain the concentrationof recombination centers. From the obtained concentration ofrecombination centers, the regions in the measurement sample areinferred.

A second aspect of the present invention pertains to second and fourthidentification methods, and it is a method of identifying a point defectdistribution in a silicon single crystal ingot, including the steps of:(a) slicing a first silicon single crystal ingot in an axial directionthereof, the first silicon single crystal ingot pulled from a firstsilicon melt at a varying pull rate, to prepare first and secondreference samples each including a region [V], a region [Pv], a region[Pi], and a region [I]; (b) coating each of surfaces of the first andsecond reference samples with first and second transition metalsolutions in which respective different transition metals are dissolvedat a concentration of 1 to 1000 ppm to metal-contaminate the referencesamples; (c) heat-treating the metal-contaminated first and secondreference samples in an atmosphere of argon, nitrogen, oxygen, hydrogen,or a mixed gas thereof either at temperatures of 600 to 1200° C. for 0.5to 24 hours while increasing the temperature at a rate of 0.5 to 10°C./minute, or at temperatures of 600 to 1100° C. for 10 to 60 secondswhile increasing the temperature at a rate of 30 to 70° C./second, todiffuse the transition metals in the first and second reference samples;(d) measuring a concentration of recombination centers formed by thetransition metal in the entire heat-treated first reference sample; (e)measuring recombination lifetimes associated with the transition metalin the entire heat-treated second reference sample; (f) producing acorrelation line between the concentration of recombination centers andthe recombination lifetimes from measurement results obtained in thestep (d) and the step (e), and defining regions including at least theregion [Pv] and the region [Pi] as well as a boundary thereof in thefirst reference sample; (g) slicing a second silicon single crystalingot in an axial direction thereof, the second silicon single crystalingot pulled from a second silicon melt at a predetermined pull rate, toprepare a measurement sample including at least a region [Pv] and aregion [Pi]; (h) coating a surface of the measurement sample with athird transition metal solution that is the same as the secondtransition metal solution to metal-contaminate the measurement sample;(i) heat-treating the metal-contaminated measurement sample under thesame conditions as those in the step (c) to diffuse the transition metalin the measurement sample; (j) measuring a recombination lifetimeassociated with the transition metal in the entire heat-treatedmeasurement sample; and (k) checking results of the measuring in thestep (j) against the correlation line to infer the region [Pv] and theregion [Pi] as well as a boundary thereof.

The sliced first and second reference samples and the measurement sampleare such that an oxygen concentration thereof is within the range of8.0×10¹⁷ to 1.0×10¹⁸ atoms/cm³, or a boundary between the region [Pv]and the region [Pi] is unidentifiable in the samples when theirrecombination lifetimes are measured after the samples are heat-treatedat 800° C. for 4 hours in a nitrogen atmosphere and subsequently furtherheat-treated at 1000° C. for 16 hours. The definitions of the region[V], the region [Pv], the region [Pi], and the region [I] are the sameas those set forth in the foregoing first aspect.

A third aspect of the present invention pertains to fifth identificationmethod, and it is a method of identifying a point defect distribution ina silicon single crystal ingot, including the steps of: (a) slicing asilicon single crystal ingot in an axial direction thereof, the ingotpulled from a silicon melt at a varying pull rate, to prepare first andsecond samples each including a region [V], a region [Pv], a region[Pi], and a region [I]; (b) measuring oxygen concentrations of the firstand second samples; (c) subjecting the first sample to a first heattreatment at 800° C. for 4 hours in a nitrogen or oxidizing atmosphereand subsequently to a second heat treatment at 1000° C. for 16 hours,when the oxygen concentrations of the first and second samples are1.2×10¹⁸ atoms/cm³ or higher; (d) measuring recombination lifetimes inthe entire heat-treated first sample; (e) defining a boundary betweenthe region [Pi] and the region [I] in the first sample based onmeasurement results in the step (d); (f) subjecting the second sample toa third heat treatment at 1100 to 1200° C. for 1 to 4 hours in anoxidizing atmosphere; (g) selectively etching the second samplesubjected to the third heat treatment; (h) observing theselectively-etched second sample with an optical microscope to identifyan oxidation induced stacking fault (OISF) region; and (i) defining aboundary between the region [V] and the region [Pv] in the second samplebased on a result of the observing in the step (h).

The definitions of the region [V], the region [Pv], the region [Pi], andthe region [I] are the same as those set forth in the foregoing firstaspect.

In accordance with the third aspect of the invention, employing theabove-described method makes it possible to easily identify the boundarybetween the region [Pv] and the region [V] in an ingot from thedistribution of OISF even when the concentration of oxygen dissolved inthe ingot is a high concentration of 1.2×10¹⁸ atoms/cm³ or higher.

A fourth aspect of the present invention pertains to a sixthidentification method, and it is a method of identifying a point defectdistribution in a silicon single crystal ingot, including the steps of:(a) slicing a silicon single crystal ingot in an axial directionthereof, the ingot pulled from a silicon melt at a varying pull rate, toprepare first and second samples each including a region [V], a region[Pv], a region [Pi], and a region [I]; (b) measuring oxygenconcentrations of the first and second samples; (c) subjecting the firstsample to a third heat treatment at 1100 to 1200° C. for 1 to 4 hours inan oxidizing atmosphere when the oxygen concentrations of the first andsecond samples are 9.0×10¹⁷ atoms/cm³ or lower; (d) selectively etchingthe first sample that has been subjected to the third heat treatment;(e) observing the selectively-etched first sample with an opticalmicroscope to identify an oxidation induced stacking fault (OISF)region; (f) defining a boundary between the region [V] and the region[Pv] in the first sample based on a result of the observing in the step(e); (g) selectively etching the second sample; (h) observing theselectively-etched second sample with an optical microscope to identifyan interstitial-type large dislocation region; and (i) defining aboundary between the region [Pi] and the region [I] in the second samplebased on a result of the observing in the step (h).

The definitions of the region [V], the region [Pv], the region [Pi], andthe region [I] are the same as those set forth in the foregoing firstaspect.

A fifth aspect of the present invention pertains to a seventhidentification method, and it is a method of identifying a point defectdistribution in a silicon single crystal ingot, including the steps of:(a) slicing a silicon single crystal ingot in an axial directionthereof, the ingot pulled from a silicon melt at a varying pull rate, toprepare first and second samples each including a region [V], a region[Pv], a region [Pi], and a region [I]; (b) measuring oxygenconcentrations of the first and second samples; (c) subjecting the firstsample to a first heat treatment at 800° C. for 4 hours in a nitrogen oroxidizing atmosphere and subsequently to a second heat treatment at1000° C. for 16 hours, when the oxygen concentrations of the first andsecond samples are 9.0×10¹⁷ atoms/cm³ or less; (d) measuringrecombination lifetimes in the entire heat-treated first sample; (e)defining a boundary between the region [Pi] and the region [I] and aboundary between the region [V] and the region [Pv] in the first samplebased on measurement results in the step (d); (f) subjecting the secondsample to a fourth heat treatment at 700° C. to not more than 800° C.for 4 to 20 hours or at 800° C. for more than 4 to 20 hours in anitrogen or oxidizing atmosphere and subsequently to a fifth heattreatment at 1000° C. for 1 to 20 hours; (g) measuring recombinationlifetimes in the entire heat-treated second sample; and (h) defining aboundary between the region [Pi] and the region [I] and a boundarybetween the region [V] and the region [Pv] in the second sample based onmeasurement results in the step (g).

The definitions of the region [V], the region [Pv], the region [Pi], andthe region [I] are the same as those set forth in the foregoing firstaspect.

In accordance with the fourth and fifth aspects of the invention,employing the above-described methods makes it possible to easilyidentify the boundary between the region [V] and the region [Pv] and theboundary between the region [Pi] and the region [I] even when the oxygendissolved in an ingot is a low concentration of 9.0×10¹⁷ atoms/cm³ orlower.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart pertaining to a measurement sample in a firstidentification method according to the present invention;

FIG. 2 a flowchart pertaining to a reference sample in the firstidentification method according to the present invention;

FIG. 3 illustrates how samples are prepared from an ingot;

FIG. 4 is a figure showing recombination lifetime in an entire samplemeasured by an LM-PCD method after a pilot sample having a high oxygenconcentration is subjected to a two-stage heat treatment, according tothe first identification method;

FIG. 5 is a figure showing recombination lifetime in an entire samplemeasured by an LM-PCD method after a measurement sample having a lowoxygen concentration is subjected to Fe contamination and heattreatment, according to the first identification method;

FIG. 6 is a graph showing the concentration of recombination centersassociated with Fe in a reference sample having a low oxygenconcentration when the sample is measured using a DLTS method, inaccordance with the first identification method;

FIG. 7 is a graph showing the recombination lifetime associated with Fein the entire reference sample having a low oxygen concentration whenthe sample is measured by an LM-PCD method, in accordance with the firstidentification method;

FIG. 8 is a graph in which FIG. 6 and FIG. 7 are combined;

FIG. 9 is a graph showing a calibration line obtained by plotting themeasurement values obtained by the DLTS method and the reciprocals ofthe measurement values obtained by the LM-PCD method for the referencesample having a low oxygen concentration, in accordance with the firstidentification method;

FIG. 10 is a flowchart pertaining to a measurement sample in a secondidentification method according to the present invention;

FIG. 11 is a flowchart pertaining to reference samples in the secondidentification method according to the present invention;

FIG. 12 is a flowchart pertaining to a measurement sample in a thirdidentification method according to the present invention;

FIG. 13 is a flowchart pertaining to a reference sample in the thirdidentification method according to the present invention;

FIG. 14 is a flowchart pertaining to a measurement sample in a fourthidentification method according to the present invention;

FIG. 15 is a flowchart pertaining to reference samples in the fourthidentification method according to the present invention;

FIG. 16 is a flowchart pertaining to measurement samples in a fifthidentification method according to the present invention;

FIG. 17 is a flowchart pertaining to measurement samples in a sixthidentification method according to the present invention;

FIG. 18 is a flowchart pertaining to measurement samples in a seventhidentification method according to the present invention;

FIG. 19 illustrates how the measurement samples are prepared from a testingot;

FIG. 20(a) is a figure showing a recombination lifetime in the entirefirst sample that has undergone a second heat treatment in the fifthidentification method, and FIG. 20(b) is a figure showing arecombination lifetime in the entire second sample that has undergone athird heat treatment in the fifth identification method;

FIG. 21(a) is a figure showing a recombination lifetime in the entirefirst sample that has undergone a third heat treatment in the sixthidentification method, and FIG. 21(b) is a figure showing a L/Ddistribution in the entire second sample in the sixth identificationmethod;

FIG. 22 is a flowchart pertaining to a measurement sample in aconventional identification method;

FIG. 23 is a view showing a recombination lifetime in an entire samplewhen the conventional identification method is performed; and

FIG. 24 is a graph showing the relationship between V/G and point defectconcentration according to Voronkov's theory, in which the horizontalaxis shows V/G and the vertical axis shows, on the same axis, theconcentration of vacancy-type point defects and the concentration ofinterstitial silicon-type point defects.

DETAILED DESCRIPTION OF THE INVENTION

(1) Ingots that are the Subject of Identification Methods According tothe Present Invention

The ingots that are the subject of first to fourth identificationmethods according to the present invention are pulled while controllinga V/G ratio such that the ingots include at least the region [Pv] andthe region [Pi]. In addition, these ingots are either such ingots thattheir oxygen concentration is in the range of 8.0×10¹⁷ to 1.0×10¹⁸atoms/cm³ (old ASTM, likewise hereinafter), or such ingots that theboundary between the region [Pv] and the region [Pi] cannot beidentified in the samples sliced from these ingots when recombinationlifetimes are measured after the samples are heat-treated at 800° C. for4 hours in a nitrogen atmosphere and subsequently further heat-treatedat 1000° C. for 16 hours.

The ingots that are the subject of a fifth identification methodaccording to the present invention are pulled while controlling a V/Gratio such that the ingots include the region [V], the region [Pv], theregion [Pi], and the region [I]. In addition, these ingots are eithersuch ingots that their oxygen concentration is 1.2×10¹⁸ atoms/cm³ (oldASTM, likewise hereinafter) or higher, or such ingots that the boundarybetween the region [V] and the region [Pv] in the samples sliced fromthese ingots cannot be identified when recombination lifetime aremeasured after the samples are heat-treated at 800° C. for 4 hours in anitrogen or oxidizing atmosphere and subsequently further heat-treatedat 1000° C. for 16 hours.

The ingots that are the subject of sixth and seventh identificationmethods according to the present invention are pulled while controllinga V/G ratio such that the ingots contain the region [V], the region[Pv], the region [Pi], and the region [I]. In addition, these ingots areeither such ingots that their oxygen concentration is 9.0×10¹⁷ atoms/cm³(old ASTM, likewise hereinafter) or lower, or such ingots that theboundary between the region [Pi] and the region [I] cannot be identifiedin the samples sliced from these ingots when the recombination lifetimeof the samples are measured after the samples are heat-treated at 800°C. for 4 hours in a nitrogen or oxidizing atmosphere and subsequentlyfurther heat-treated at 1000° C. for 16 hours.

These ingots are pulled from a silicon melt in a hot-zone furnace usinga CZ method or an MCZ method according to Voronkov's theory. Generally,when a silicon single crystal ingot is pulled from a silicon melt in ahot zone furnace using a CZ method or an MCZ method, point defects andagglomerates of point defects (three-dimensional defects) are caused asdefects in a silicon single crystal. Of the point defects, there are twocommon types, vacancy-type point defect and interstitial silicon-typepoint defect. In a vacancy-type point defect, one of the silicon atomsis detached from one of their normal positions in a silicon crystallattice. Agglomerated vacancies of this kind form vacancy-type defects.On the other hand, interstitial silicons that have agglomerated forminterstitial silicon-type defects.

Generally, point defects are introduced at a contact surface between asilicon melt (molten silicon) and an ingot (solid silicon). However, asthe ingot is pulled, the portion that has been the contact surface isgradually cooled while the ingot is continuously pulled. During thecooling, vacancies or interstitial silicons undergo diffusion and a pairannihilation reaction. At the time when cooled to about 1100° C.,excessive point defects form agglomerates of vacancy-type defects(vacancy agglomerates) or agglomerates of interstitial silicon-typedefects (interstitial agglomerates). In other words, they arethree-dimensional structures originated from excessive point defectsforming agglomerates.

Agglomerates of vacancy-type defects include the defects referred to asLSTD (Laser Scattering Tomograph Defects) or FPD (Flow Pattern Defects),other than the previously-described COP. The agglomerates ofinterstitial silicon-type defects include defects referred to as L/D,which have been mentioned previously. FPD is a source of tracesexhibiting a peculiar flow pattern, which is exposed when a siliconwafer prepared by slicing an ingot is subjected to Secco etching for 30minutes (Secco etching is an etching using a mixed solution ofHF:K₂Cr₂O₇ (0.15 mol/l) 2:1). LSTD has a different index of refractionfrom that of silicon and becomes a source of generating scattered lightwhen the interior of a silicon single crystal is irradiated withinfrared rays.

According to Voronkov's theory, ratio V/G (mm²/minute·° C.), where thepull rate for an ingot is V (mm/minute) and the axial temperaturegradient in the vicinity of the solid-liquid interface between a siliconmelt and a silicon ingot is G (° C./mm), is controlled so as to grow ahigh purity ingot with a small number of defects. In this theory, therelationship between V/G and the concentration of point defects isgraphically illustrated by showing V/G on the horizontal axis andshowing the concentration of vacancy-type defects and the concentrationof interstitial silicon-type defects on the same vertical axis, asdepicted in FIG. 24, and thus, it is explained that the boundary betweena vacancy region and an interstitial silicon region is determined byV/G. More specifically, when the V/G ratio is higher than the criticalpoint (V/G)_(c), the concentration of vacancies increases in theresulting ingot; in contrast, when the V/G ratio is lower than thecritical point (V/G)_(c), the concentration of intersititial siliconsincreases in the resulting ingot. In FIG. 24, [I] represents a region inwhich the interstitial silicons are predominant and the agglomerateddefects of interstitial silicons are contained ((V/G)₁ or lower), [V]represents a region in which vacancies are predominant and theagglomerated defects of vacancies are contained ((V/G)₂ or higher), [P]is a perfect region in which neither the agglomerates of vacancy-typedefects nor the agglomerates of interstitial silicon-type defects exist((V/G)₁ to (V/G₂)). In the boundary of the region [V] that is adjacentto the region [P], there exists a P band region ((V/G)₂ to (V/G)₃), inwhich OISF nuclei are formed. In the P band region, there exist microplate-shaped precipitates and OISF (stacking fault) is formed by a heattreatment in an oxidizing atmosphere. Likewise, in the boundary of theregion [I] that is adjacent to the region [P], there exists a B bandregion. In the B band region, micro dislocation clusters exist at a highdensity.

The perfect region [P] is further classified into a region [Pi] and aregion [Pv]. The region [Pi] is a region in which the V/G ratio is fromthe above-mentioned (V/G)₁ to the critical point, and in whichinterstitial silicons are predominant and their agglomerated defects arenot contained. The region [Pv] is a region in which the V/G ratio isfrom the critical point to the above-mentioned (V/G)₂, and in whichvacancies are predominant and their agglomerated defects are notcontained. In other words, [Pi] is a region that is adjacent to theregion [I] and has a concentration of interstitial silicons less thanthe minimum concentration of interstitial silicons at which aninterstitial-type large dislocation can be formed, and [Pv] is a regionthat is adjacent to the region [V] and has a concentration of vacanciesless than the minimum concentration of vacancies defects at which anOISF nucleus can be formed.

The ingots that are the subject of the first to the fourthidentification methods according to the present invention are pulledwhile controlling a V/G ratio to be within the range of (V/G)₁ to(V/G)₂.

The ingots that are the subject of the methods according to fifth toseventh identification method according to the present invention arepulled while controlling a V/G ratio so as to include each of theabove-mentioned regions of from (V/G)₁ or lower to the critical point,and from the critical point to (V/G)₃ or above, in order to contain theregion [V], the region [Pv], the region [Pi], and the region [I].

(2) First Identification Method According to the Present Invention

Next, with reference to FIGS. 1 to 3, a first identification methodaccording to the present invention is described. First, as shown inFIGS. 2 and 3, a pilot sample and a reference sample are prepared froman ingot. Specifically, a first ingot is pulled from a first siliconmelt stored in a quartz crucible of pulling equipment according to a CZmethod or an MCZ method. In this pulling of the ingot, pull rate V(mm/minute) is varied from a high rate (top side) to a low rate (bottomside) so that the ingot contains the region [V], the region [Pv], theregion [Pi], and the region [I] along the axial direction. The obtainedfirst ingot is sliced in the axial direction and subjected to a mirroretching to prepare a pilot sample and a reference sample in wafer shapehaving a thickness of 500 to 1000 μm and a mirror-finished surface.

The concentration of oxygen dissolved in the pilot sample is measuredusing an FT-IR (Fourier transform infrared absorption spectroscopy)method. When the oxygen concentration is 1.0×10¹⁸ atoms/cm³ or higher,the pilot sample is heat-treated at 800° C. for 4 hours in a nitrogenatmosphere, and subsequently further heat-treated at 1000° C. for 16hours. The recombination lifetime of the heat-treated sample is measuredover the entire sample using an LM-PCD method. FIG. 4 shows an exampleof the result of the measurement. As shown in FIG. 4, oxygenprecipitates appear in the sample at a high density as a result of theabove-described two-stage heat treatment. According to the distributionof their concentrations, the region [V], the region [Pv], the region[Pi], and the region [I] are clearly identified in the entire sample. Inthe region [V], there exists a P band region in which OISF nuclei areformed, and in the region [I], there exists a B band region.

On the other hand, when the oxygen concentration is less than 1.0×10¹⁸atoms/cm³ in the pilot sample, it is impossible to identify, as seenfrom FIG. 5, whether a perfect region [P] is a region [Pv] or a region[Pi] based on the result of the recombination lifetime measurement inthe same manner as described above in which the sample is heat-treatedand the recombination lifetime is measured. In view of this, in thepresent invention, the reference sample is metal-contaminated by coatingthe sample with a transition metal solution, as shown in FIG. 2.Examples of the transition metal include Fe, Ni, Cu, or Co. Thetransition metal solution is a solution in which the transition metal isdissolved at a concentration of 1 to 1000 ppm, preferably, 1 to 100 ppm.Especially preferred is a standard solution for atomic absorptionspectroscopy in which Fe, Ni, Cu, or Co is dissolved at a concentrationof 10 to 100 ppm. Examples of the coating method include a spin coatingmethod and a dipping method. When the concentration of the transitionmetal solution is less than the lower limit value, the subsequentmeasurement of recombination lifetime cannot be made with highprecision. On the other hand, when the concentration exceeds the upperlimit value, the transition metal diffuses over the front and backsurfaces of the sample, forming metal silicide (precipitate) in thevicinity of the front and back surfaces; as a result, it is impossibleto form a recombination center, which is a deep energy level, inside thesample (in the crystal).

In the first identification method, as shown in FIG. 2, the referencesample is Fe-contaminated by coating a surface thereof with a solutionin which Fe is dissolved at a concentration of 1 ppm by a spin coatingmethod. The Fe-contaminated reference sample is heat-treated in anatmosphere of argon, nitrogen, oxygen, hydrogen, or a mixed gas thereof.For this heat treatment, there are two possible methods; one is a methodin which the heat treatment is carried out at temperatures of 600 to1200° C. for 0.5 to 24 hours while increasing the temperature at a rateof 0.5 to 10° C./minite, and the other is a method in which the heattreatment is carried out at temperatures of 600 to 1100° C. for 10 to 60seconds while increasing the temperature at a rate of 30 to 70°C./second (RTA method). For the former method, it is preferable to carryout the heat treatment at 900 to 1000° C. for 1 to 2 hours whileincreasing the temperature at a rate of 5 to 10° C./minute. For thelatter method, (RTA method), it is preferable to carry out the heattreatment at 900 to 1000° C. for 30 to 60 seconds while increasing thetemperature at a rate of 30 to 50° C./second. When the duration and thetemperature of the heat treatment is less than the lower limit values,the transition metal does not diffuse in the sample sufficiently. On theother hand, when they exceed the upper limit values, the transitionmetal diffuses over the front and back surfaces of the sample, formingmetal silicide (precipitate) in the vicinity of the front and backsurfaces, and therefore, it is impossible to form a recombinationcenter, which is a deep energy level, in the sample (in the crystal).

The reference sample that has undergone the heat treatment is subjectedto the following two measurements. Specifically, they include (A) ameasurement of the concentration of recombination centers in the entiresample that are formed by the transition metal, and (B) a measurement ofrecombination lifetimes associated with the transition metal. For themethod of the above (A), a DLTS (Deep Level Transient Spectroscopy)method is preferable, whereas for the method of the above (B), an LM-PCD(Laser/Microwave Photoconductivity Decay) method is preferable. The DLTSmethod quantifies the concentration of recombination centers formed bythe transition metal in the sample. The LM-PCD method quantifies therecombination lifetimes over the entire sample.

FIG. 6 shows the result of the measurement of the concentration ofrecombination centers obtained by the above (A). FIG. 7 shows the resultof the measurement of recombination lifetimes obtained by the above (B).In FIGS. 6 and 7, their horizontal axes correspond to the axialdirection of the ingot from which the samples are not yet sliced. TheV/G ratio is large at the right end of the horizontal axis in bothfigures, and the V/G ratio is small at the left end thereof. From FIG.6, it is understood that while varying the V/G ratio, Fe concentrationis the highest in a central region R₁ of the sample, and that Feconcentration is the lowest in a region R₂ that is on the right of theregion R1. Likewise, from FIG. 7, it is understood that deterioration ofthe recombination lifetime is very severe in a central region R₃ of thesample, and that the recombination lifetime is the highest in a regionR₄ that is on the right of the region R₃. FIG. 8 shows a graph in whichFIG. 6 and FIG. 7 are combined. From FIG. 8, it is understood that theregion R₁ is in agreement with R₃, and the region R₂ is in agreementwith R₄. From the result of FIG. 4 and Voronkov's theory, it isconcluded that the region R₁ (R₃) is a region [Pi] in which oxygenprecipitation is suppressed most, and it is concluded that the region R₂(R₄) is a region [Pv] in which oxygen precipitation is promoted most.

The reason that the region exhibiting the highest Fe concentration isthe region [Pi] and the region exhibiting the lowest Fe concentration isthe region [Pv] is as follows. When a solution of Fe, which is atransition metal, is coated on a sample surface and Fe is diffused inthe sample by a heat treatment, Fe is captured by vacancies, and whenthe sample is cooled, oxygen precipitates including Fe are formed in theregion [V] and the region [Pv] within the sample. Since the content ofFe in the oxygen precipitates is attributed to the amount ofvacancy-type point defects, the content of Fe in the region [Pv] is lessthan that in the region [V]. On the other hand, since the region [Pi]contains neither vacancies that capture transition metal noragglomerated interstitial silicon-type point defects, Fe stays in thebulk of silicon. In the region [I], Fe reacts with agglomeratedinterstitial silicon-type point defects, forming Fe silicide. Theabove-described behavior of Fe in each of the regions is consistent anddoes not depend on whether the concentration of oxygen is high or low.By thus measuring the concentration of recombination centers formed byFe groups in the entire sample, it is possible to define the region [Pv]and the region [Pi] as well as the boundary thereof in the sample.

Meanwhile, it is inferred from FIG. 8 that the recombination lifetimeand the concentration of recombination centers correlate with eachother, and FIG. 9 is obtained by plotting the measurement values of theabove (A) and reciprocals of the measurement values of the above (B). Itis understood from FIG. 9 that the two lines are in a linearcorrelation, and a calibration line is produced from this linear line ofcorrelation. This linear line is represented by the following Equation(1):Fe=(1×10¹³/lifetime) (cm⁻³·μs)  (1)FIG. 9 also shows the region [Pv], the region [Pi], and the region [I].

Next, as shown in FIG. 1, a pilot sample and a measurement sample areprepared from a measurement ingot. Specifically, a second ingot ispulled from a second silicon melt stored in a quartz crucible of pullingequipment according to a CZ method or an MCZ method. In the pulling ofthe second ingot, for example, a V/G value is controlled to be withinthe range of (V/G)₁ to (V/G)₂ shown in FIG. 24 so that the second ingotcontains the perfect region [P] over the entire length. The obtainedsecond ingot is sliced in the axial direction and subjected to a mirroretching to prepare a pilot sample and a measurement sample in wafershape having a thickness within the range of 0.5 to 1 mm and amirror-finished surface.

The concentration of the oxygen dissolved in the pilot sample ismeasured using an FT-IR method, and when the oxygen concentration is1.0×10¹⁸ atoms/cm³ or higher, the same two-stage heat treatment as theabove-described two-stage heat treatment for the reference sample iscarried out. The recombination lifetime of the heat-treated sample ismeasured over the entire sample using the LM-PCD method. In a pilotsample having a high oxygen concentration, oxygen precipitates appear inthe sample at a high density due to the two-stage heat treatment, andtherefore, the region [Pv] and the region [Pi] as well as the boundarythereof are clearly identified in the sample.

On the other hand, when the oxygen concentration of the pilot sample isless than 1.0×10¹⁸ atoms/cm³, the measurement sample shown in FIG. 1 iscoated with the transition metal solution to metal-contaminate thissample in a similar manner to the metal contamination for the referencesample. In the first identification method, as shown in FIG. 1, themeasurement sample is Fe-contaminated by coating a surface thereof witha solution in which Fe is dissolved at a concentration of 1 ppm. TheFe-contaminated measurement sample is subjected to a similar heattreatment to that for the above-described reference sample. Therecombination lifetime associated with Fe is measured regarding theheat-treated measurement sample. By applying a measurement value of therecombination lifetime to the calibration line shown in FIG. 9 andaccordingly obtaining the concentration of recombination centers, it ispossible to infer whether a region in the measurement sample is theregion [Pv] or the region [Pi], from the obtained value of theconcentration of recombination centers.

(3) Second Identification Method According to the Present Invention

A second identification method is as shown in FIGS. 10 and 11. As shownin FIG. 11, when the oxygen concentration of the pilot sample is lessthan 1.0×10¹⁸ atoms/cm³, two reference samples are sliced from theingot. A first reference sample is coated with a solution in which Fe isdissolved to Fe-contaminate the sample, while a second reference sampleis coated with a solution in which Ni or Co is dissolved to Ni- orCo-contaminate the sample. These reference samples are separately orsimultaneously heat-treated. The conditions in the heat treatment arethe same as those in the first identification method.

Regarding the first reference sample, which has been Fe-contaminated andthereafter heat-treated, the concentration of recombination centersassociated with Fe is measured in a similar manner to that in the firstidentification method. Thereby, the region [V], the region [Pv], theregion [Pi], and the region [I] can be identified in the first referencesample having a low oxygen concentration. Meanwhile, the recombinationlifetime is measured regarding the second reference sample that has beenNi- or Co-contaminated and thereafter heat-treated, in a similar mannerto that in the first identification method. Subsequently, the secondreference sample is etched using an etchant solution including NH₃, HF,and CH₃COOH. Ni or Co has a greater diffusion coefficient than Fe andtherefore precipitates in the vicinity of the sample surface due to theheat treatment, but by this etching, Ni or Co present in the vicinity ofthe surface is removed. Under this condition, the recombination lifetimein the second reference sample is measured again in a similar manner tothat in the first identification method. Then, the difference betweenthe recombination lifetimes in the entire sample before and after thechemical etching is computed. As shown in FIG. 11, it is understood thatafter the chemical etching, the region [Pi] and the region [I] showremarkable increases in their recombination lifetimes, whereas theregion [V] does not show a change in the recombination lifetime beforeand after the chemical etching. For this reason, it is also possible toidentify the region [V], the region [Pv], the region [Pi], and theregion [I] with the use of Ni diffusion from the amount of recombinationlifetime increased before and after the chemical etching.

In a similar manner to that in the first identification method, acalibration line (not shown) is produced from the correlation betweenthe measurement values of the concentrations of recombination centersassociated with Fe in the first reference sample and the measurementvalues of the recombination lifetimes in the second reference sample.

Next, as shown in FIG. 10, a pilot sample and a measurement sample areprepared from a measurement ingot in a similar manner to that in firstidentification method. This ingot is pulled while controlling a V/Gvalue so as to be within the range of (V/G)₁ to (V/G)₂ shown in FIG. 24so that it contains the perfect region [P] over the entire length. Whenthe oxygen concentration of this pilot sample is less than 1.0×10¹⁸atoms/cm³, as in the second reference sample, the measurement sample iscoated with a Ni or Co solution to Ni- or Co-contaminate this sample.This Ni- or Co-contaminated measurement sample is heat-treated similarlyto the foregoing second reference sample. As shown in FIG. 10, theheat-treated measurement sample is subjected to a measurement ofrecombination lifetimes associated with Ni or Co. After chemicallyetching the measurement sample similarly to the second reference sample,the recombination lifetimes are measured again regarding the measurementsample. Subsequently, the difference between the recombination lifetimesin the entire sample before and after the chemical etching is computed.By applying a measurement value of the recombination lifetime to thecalibration line, which is not shown in the figure, it is possible toinfer whether a region in the measurement sample is the region [Pv] orthe region [Pi].

(4) Third Identification Method According to the Present Invention

A third identification method is as shown in FIGS. 12 and 13. As shownin FIG. 13, a pilot sample is first subjected to the same two-stage heattreatment as that in the first identification method, and therecombination lifetime is measured. When the boundaries of the region[V], the region [Pv], the region [Pi], and the region [I] cannot beidentified in the pilot sample as a result of this, a reference samplesliced from the ingot is coated with a solution in which Fe is dissolvedto Fe-contaminate the sample. This reference sample is heat-treatedsimilarly to the first identification method. The concentration ofrecombination centers associated with Fe is measured in the heat-treatedreference sample similarly to the first identification method, it ispossible to identify the region [V], the region [Pv], the region [Pi],and the region [I] in the reference sample. Meanwhile, the recombinationlifetime of the reference sample that has been Fe-contaminated andthereafter heat-treated is measured similarly to the firstidentification method. From the correlation between the measurementvalues of the concentration of recombination centers associated with Feand the measurement values of the recombination lifetime in thereference sample, a calibration line, although not shown in the figure,is produced similarly to that in the first identification method.

Next, as shown in FIG. 12, a pilot sample and a measurement sample areprepared from a measurement ingot, similarly to those in the firstidentification method. This ingot is pulled while controlling a V/Gvalue to be within the range of (V/G)₁ to (V/G)₂ shown in FIG. 24 suchthat it contains a perfect region [P] over the entire length. This pilotsample is subjected to the same two-stage heat treatment as that in thefirst identification method, and the recombination lifetime is measured.When the region [Pv] and the region [Pi] as well as the boundary thereofcannot be identified in the pilot sample as a result of this, themeasurement sample is coated with a solution in which Fe is dissolved toFe-contaminate the sample. After this measurement sample is heat-treatedsimilarly to that in first identification method, the recombinationlifetime associated with Fe is measured in the measurement sample. Byapplying a measurement value of the recombination lifetime to thecalibration line, which is not shown in the figure, and accordinglyobtaining the concentration of recombination centers, it is possible toinfer whether a region in the measurement sample is the region [Pv] orthe region [Pi] from the obtained value of the concentration ofrecombination centers.

(5) Fourth Identification Method According to the Present Invention

A fourth identification method is shown in FIGS. 14 and 15. As shown inFIG. 15, a pilot sample is first subjected to the same two-stage heattreatment as that in the first identification method, and therecombination lifetime is measured. When the boundaries of the region[V], the region [Pv], the region [Pi], and the region [I] cannot beidentified in the pilot sample as a result of this, two referencesamples are sliced from an ingot, and a first reference sample is coatedwith a solution in which Fe is dissolved to Fe-contaminate the sample,whereas a second reference sample is coated with a solution in which Nior Co is dissolved to Ni- or Co-contaminate the sample. These referencesamples are separately or simultaneously heat-treated. The conditions ofthe heat treatment are the same as those in the first identificationmethod.

For the first reference sample that has been Fe-contaminated andthereafter heat-treated, the concentration of recombination centersassociated with Fe is measured in a similar manner to that in the firstidentification method. Thereby, the region [V], the region [Pv], theregion [Pi], and the region [I] can be identified in the first referencesample. Meanwhile, the second reference sample that has been Ni- orCo-contaminated and thereafter heat-treated is chemically etched in asimilar manner to that in the second identification method, andthereafter, the recombination lifetime of the second reference sample ismeasured again. Subsequently, the difference between the recombinationlifetimes in the entire sample before and after the chemical etching iscomputed. From the correlation between the measurement values of theconcentration of recombination centers associated with Fe in the firstreference sample and the measurement values of the recombinationlifetimes in the second reference sample, a calibration line, althoughnot shown in the figure, is produced similarly to that in the firstidentification method.

Next, as shown in FIG. 14, a pilot sample and a measurement sampleprepared from a measurement ingot in a similar manner to that in thefirst identification method. This ingot is pulled while controlling aV/G value so as to be within the range of (V/G)₁ to (V/G)₂ shown in FIG.24 such that a perfect region [P] is contained over the entire length.This pilot sample subjected to the same two-stage heat treatment as thatin the first identification method, and the recombination lifetime ismeasured. When the region [Pv] and the region [Pi] as well as theboundary thereof cannot be identified in the pilot sample as a result ofthis, the measurement sample is coated with a Ni or Co solution to Ni-or Co-contaminate this sample as in the case of the second referencesample. This Ni- or Co-contaminated measurement sample is heat-treatedas in a similar manner to the second reference sample. The recombinationlifetime associated with Ni or Co is measured in the heat-treatedmeasurement sample. This measurement sample is also chemically etched asis the second reference sample, and the recombination lifetime of themeasurement sample is measured again. Subsequently, the differencebetween the recombination lifetimes in the entire sample before andafter the chemical etching is computed. By applying a measurement valueof the recombination lifetime to the calibration line, although notshown in the figure, it is possible to infer whether a region in themeasurement sample is the region [Pv] or the region [Pi].

(6) Fifth Identification Method According to the Present Invention

Next, with reference to FIGS. 16 and 19, a fifth identification methodof the present invention is described.

As shown in FIG. 16, first and second measurement samples are preparedfrom a test ingot at first. Specifically, an ingot is pulled from asilicon melt stored in a quartz crucible of pulling equipment accordingto a CZ method or an MCZ method. In pulling the ingot, a pull rate V(mm/minute) of the ingot is varied either from a high rate (top side) toa low rate (bottom side) or from a low rate (bottom side) to a high rate(top side) so that the region [V], the region [Pv], the region [Pi], andthe region [I] are contained along the axial direction of the ingot.Subsequently, as shown in FIG. 19, the ingot obtained in the test issliced in the axial direction and subjected to a mirror etching, wherebya measurement sample having a thickness of 500 to 2000 μm and amirror-finished surface is prepared.

Next, returning to FIG. 16, the concentrations of oxygen dissolved inthe first and second samples are measured using an FT-IR (Fouriertransform infrared absorption spectroscopy) method. When the oxygenconcentrations of the first and second samples are 1.2×10¹⁸ atoms/cm³ orhigher, the first sample is subjected to a first heat treatment at 800°C. for 4 hours in a nitrogen or oxidizing atmosphere and subsequentlysubjected to a second heat treatment at 1000° C. for 16 hours. When theconcentrations of oxygen dissolved in the first and second samples fallout of the foregoing range, an identification method is carried out asfollows. When the oxygen concentration is 9.0×10¹⁷ atoms/cm³ or lower, alater-described sixth identification method or a later-described seventhidentification method is used. When the oxygen concentration is in therange of less than 1.2×10¹⁸ atoms/cm³ and greater than 9.0×10¹⁷atoms/cm³, the recombination lifetime is measured after performing firstand second heat treatments, as have been conventionally carried out, asshown in FIG. 22, whereby the boundary between the region [V] and theregion [Pv] as well as the boundary between the region [Pi] and theregion [I] can be clearly distinguished, as shown in FIG. 23.

In the sample having an oxygen concentration of 1.2×10¹⁸ atoms/cm³ orhigher, the boundary between the region [V] and the region [Pv] cannotbe clearly identified from the measurement result of the recombinationlifetime even when the recombination lifetime of the sample is measuredafter performing the first and second heat treatments. This is because,in the sample having an oxygen concentration of 1.2×10¹⁸ atoms/cm³ orhigher, oxygen precipitation nucleus tend to form easily and thereforethe boundary between the region [V] and the region [Pv] becomesdifficult to find.

In view of this, the identification method according to the presentinvention subjects the first sample to the first and second heattreatments, and measures the recombination lifetime of the treated firstsample to define the boundary between the region [Pi] and the region [I]as shown in FIG. 20(a). Next, the second sample is subjected to a thirdheat treatment at 1100 to 1200° C. for 1 to 4 hours in an oxidizingatmosphere. This third heat treatment exposes OISF. The second samplethat has undergone the third heat treatment is subjected to a selectiveetching without stirring. The portion that has been selectively etchedis observed with an optical microscope, and thereby, the boundary of theregion [V] and the region [Pv] can be easily identified using theoptical microscope because of the OISF distribution that is exposed, asshown in FIG. 20(b). Examples of the selective etching include Seccoetching and non-chromium etching.

Among the inflection points at the boundary between the region [Pi] andthe region [I], which are indicated by crosses (x) in FIG. 20(a), a pullrate V₂ is determined from the location of the inflection point that isclosest to the region [V]. Likewise, among the inflection points at theboundary between the region [V] and the region [Pv], which are indicatedby crosses (x) in FIG. 20(b), a pull rate V₁ is determined from thelocation of the inflection point that is closest to the region [I]. Bypulling an ingot with a pull rate being within the range of V₁ to V₂, itis possible to fabricate an ingot having a perfect region [P] in whichvacancy agglomerates and interstitial silicon agglomerates do not exist,that is, a grown-in defect-free region.

(7) Sixth Identification Method According to the Present Invention

As shown in FIG. 17, first and second measurement samples are preparedfrom a test ingot at first. Specifically, an ingot is pulled from asilicon melt stored in a quartz crucible of pulling equipment accordingto a CZ method or an MCZ method. In pulling the ingot, a pull rate V(mm/minute) of the ingot is varied either from a high rate (top side) toa low rate (bottom side) or from a low rate (bottom side) to a high rate(top side) so that the region [V], the region [Pv], the region [Pi], andthe region [I] are contained along the axial direction of the ingot.Subsequently, as shown in FIG. 19, the obtained test ingot is sliced inthe axial direction and is subjected to a mirror etching, wherebymeasurement samples having a thickness of 500 to 2000 μm and amirror-finished surface are prepared.

Next, returning to FIG. 17, the concentrations of oxygen dissolved inthe first and second samples are measured using the FT-IR (Fouriertransform infrared absorption spectroscopy) method. When the oxygenconcentrations are 9.0×10¹⁷ atoms/cm³ or lower, the first sample issubjected to a third heat treatment at 1100 to 1200° C. for 1 to 4 hoursin an oxidizing atmosphere. When the concentrations of oxygen dissolvedin the first and second samples fall out of the foregoing range, anidentification method is carried out as follows. When the oxygenconcentration is 1.2×10¹⁸ atoms/cm³ or higher, the fifth identificationmethod is performed. When the oxygen concentration is in the range ofless than 1.2×10¹⁸ atoms/cm³ and greater than 9.0×10¹⁷ atoms/cm³, thepreviously-described conventional identification method is carried out.For the sample having an oxygen concentration of 9.0×10¹⁷ atoms/cm³ orlower, the boundary between the region [Pi] and the region [I] cannot beclearly identified from the result of measuring the recombinationlifetime even when the recombination lifetime of the sample is measuredafter performing the first and second heat treatments. This is because,in the sample having an oxygen concentration of 9.0×10¹⁷ atoms/cm³ orlower, oxygen precipitation nucleus are not likely to form by performingonly the first and second heat treatments and therefore the boundarybetween the region [Pi] and the region [I] becomes difficult to find.

OISF is exposed by performing this third heat treatment. The firstsample that has undergone the third heat treatment is subjected to aselective etching. The portion that has been subjected to the selectiveetching is observed with an optical microscope, and thereby, theboundary of the region [V] and the region [Pv] can be easily identifiedusing the optical microscope because of the OISF distribution that isexposed, as shown in FIG. 21(a).

Next, the second sample is subjected to a selective etching, and byobserving the portion that has been subjected to the selective etchingwith an optical microscope, the B band region exhibiting L/D can beidentified as shown in FIG. 21(b).

Among the inflection points at the boundary between the region [V] andthe region [Pv], which are indicated by crosses (x) in FIG. 21(a), apull rate V₁ is determined from the location of the inflection pointthat is closest to the region [I]. Likewise, among the inflection pointsat the boundary between the region [Pi] and the region [I], which areindicated by crosses (x) in FIG. 21(b), a pull rate V₂ is determinedfrom the location of the inflection point that is closest to the region[V]. By pulling an ingot with a pull rate within the range of V_(1 to V)₂, it is possible to fabricate an ingot with a perfect region [P] inwhich vacancy agglomerates and interstitial silicon agglomerates do notexist, that is, a grown-in defect-free region.

(8) Seventh Identification Method according to the Present Invention

A seventh identification method is a method used at a similar oxygenconcentration to that in the sixth identification method.

As shown in FIG. 18, first and second measurement samples are preparedfrom a test ingot at first. Specifically, an ingot is pulled from asilicon melt stored in a quartz crucible of pulling equipment accordingto a CZ method or an MCZ method. In pulling the ingot, a pull rate V(mm/minute) of the ingot is varied either from a high rate (top side) toa low rate (bottom side) or from a low rate (bottom side) to a high rate(top side) so that the region [V], the region [Pv], the region [Pi], andthe region [I] are contained along the axial direction of the ingot.Subsequently, as shown in FIG. 19, the obtained test ingot is sliced inthe axial direction and is subjected to a mirror etching, wherebymeasurement samples having a thickness of 500 to 2000 μm and amirror-finished surface are prepared.

Next, returning to FIG. 18, the concentrations of oxygen dissolved inthe first and second samples are measured using the FT-IR (Fouriertransform infrared absorption spectroscopy) method. When the oxygenconcentration is 9.0×10¹⁷ atoms/cm³ or lower, the first sample issubjected to a first heat treatment at 800° C. for 4 hours in a nitrogenor oxidizing atmosphere, and subsequently is subjected to a second heattreatment at 1000° C. for 16 hours. By measuring the recombinationlifetime in the first sample, the boundary between the region [V] andthe region [Pv] as well as the boundary between the region [Pi] and theregion [I] are defined. When the boundary between the region [Pi] andthe region [I] cannot be sufficiently distinguished, the second sampleis subjected to a fourth heat treatment at 700° C. to not more than 800°C. for 4 to 20 hours or at 800° C. for more than 4 to 20 hours in anitrogen or oxidizing atmosphere and subsequently is subjected to afifth heat treatment at 1000° C. for 1 to 20 hours. When the oxygenconcentration is low and oxygen precipitation nucleus do not grow at800° C. in this way, the boundary between the region [Pi] and the region[I] can be more easily distinguished by performing the fourth and fifthheat treatments. By measuring again the recombination lifetime for thesecond sample that has undergone the fifth heat treatment, the boundarybetween the region [Pi] and the region [I] can be identified since theheat treatment condition is expanded to lower temperatures.

INDUSTRIAL APPLICABILITY

With the first to third identification methods according to the presentinvention, a region [Pv] and a region [Pi] as well as the boundarythereof within an ingot can be measured with high precision so thatpoint defect regions can be easily identified even when theconcentration of oxygen dissolved in the ingot is low. As a result, asilicon single crystal ingot having a perfect region [P] can be easilyfabricated, and a silicon wafer composed of the perfect region [P] canbe easily obtained from the ingot.

With the fifth identification method according to the present invention,the boundary between a region [Pv] and a region [V] within an ingot canbe easily identified even when the oxygen dissolved in the ingot is highin concentration. Moreover, with the sixth and seventh identificationmethods according to the present invention, the boundary between theregion [V] and the region [Pv] as well as the boundary between theregion [Pi] and the region [I] within an ingot can be easily identifiedeven in the case of low concentrations. As a result, a silicon singlecrystal ingot having a perfect region [P] can be easily fabricated, anda silicon wafer composed of the perfect region [P] can be easilyobtained from the ingot.

1. A method of identifying a point defect distribution in a siliconsingle crystal ingot, comprising the steps of: (a) slicing a firstsilicon single crystal ingot in an axial direction thereof, the ingotpulled from a first silicon melt at a varying pull rate, to prepare areference sample including a region [V], a region [Pv], a region [Pi],and a region [I]; (b) coating a surface of the reference sample with atransition metal solution in which a transition metal is dissolved at aconcentration of 1 to 1000 ppm to metal-contaminate the referencesample; (c) heat-treating the metal-contaminated reference sample in anatmosphere of argon, nitrogen, oxygen, hydrogen, or a mixed gas thereofeither at temperatures of 600 to 1200° C. for 0.5 to 24 hours whileincreasing the temperature at a rate of 0.5 to 10° C./minute, or attemperatures of 600 to 1100° C. for 10 to 60 seconds while increasingthe temperature at a rate of 30 to 70° C./second, to diffuse thetransition metal in the reference sample; (d) measuring a concentrationof recombination centers formed by the transition metal in the entireheat-treated reference sample; (e) measuring recombination lifetimesassociated with the transition metal in the entire heat-treatedreference sample; (f) producing a correlation line between theconcentration of recombination centers and the recombination lifetimesfrom measurement results obtained in the step (d) and the step (e), anddefining regions including at least the region [Pv] and the region [P]as well as a boundary thereof in the reference sample; (g) slicing asecond silicon single crystal ingot, in an axial direction thereof, thesecond silicon single crystal ingot pulled from a second silicon melt ata predetermined pull rate, to prepare a measurement sample including atleast a region [Pv] and a region [Pi]; (h) coating a surface of themeasurement sample with the same transition metal solution as thetransition metal solution to metal-contaminate the measurement sample;(i) heat-treating the metal-contaminated measurement sample under thesame conditions as those in the step (c) to diffuse the transition metalin the measurement sample; (j) measuring a recombination lifetimeassociated with the transition metal in the entire heat-treatedmeasurement sample; and (k) checking results of the measuring in thestep (j) against the correlation line to infer the region [Pv] and theregion [Pi] as well as a boundary thereof; wherein the reference sampleand the measurement sample that have been sliced are such that an oxygenconcentration thereof is within the range of 8.0×10¹⁷ to 1.0×10¹⁸atoms/cm³, or that a boundary between the region [Pv] and the region[Pi] is unidentifiable in the samples when their recombination lifetimesare measured after the samples are heat-treated at 800° C. for 4 hoursin a nitrogen atmosphere and subsequently further heat-treated at 1000°C. for 16 hours; and wherein the region [V] is a region in whichvacancy-type point defects are predominant and defects in whichexcessive vacancies are agglomerated are contained, the region [Pv] is aregion in which vacancy-type point defects are predominant and defectsin which vacancies are agglomerated are not contained, the region [Pi]is a region in which interstitial silicon-type point defects arepredominant and defects in which interstitial silicons are agglomeratedare not contained, and the region [I] is a region in which interstitialsilicon-type point defects are predominant and defects in whichinterstitial silicons are agglomerated are contained.
 2. The methodaccording to claim 1, wherein the transition metal is Fe, Ni, Cu, or Co.3. The method according to claim 1, wherein the concentration ofrecombination centers formed in the silicon single crystal by theheat-treating for diffusing the transition metal is measured using aDLTS (deep level transient spectroscopy) method.
 4. The method accordingto claim 1, wherein the recombination lifetimes subsequent to theheat-treating for diffusing the transition metal is measured using anLM-PCD (laser/microwave photoconductivity decay) method.
 5. A method ofidentifying a point defect distribution in a silicon single crystalingot, comprising the steps of: (a) slicing a first silicon singlecrystal ingot in an axial direction thereof, the first silicon singlecrystal ingot pulled from a first silicon melt at a varying pull rate,to prepare first and second reference samples each including a region[V], a region [Pv], a region [Pi], and a region [I]; (b) coating each ofthe surfaces of the first and second reference samples with a first andsecond transition metal solutions in which respective differenttransition metals are dissolved at a concentration of 1 to 1000 ppm tometal-contaminate the reference samples; (c) heat-treating themetal-contaminated first and second reference samples in an atmosphereof argon, nitrogen, oxygen, hydrogen, or a mixed gas thereof either attemperatures of 600 to 1200° C. for 0.5 to 24 hours while increasing thetemperature at a rate of 0.5 to 10° C./minute, or at temperatures of 600to 1100° C. for 10 to 60 seconds while increasing the temperature at arate of 30 to 70° C./second, to diffuse the transition metals in thefirst and second reference samples; (d) measuring a concentration ofrecombination centers formed by the transition metal in the entireheat-treated first reference sample; (e) measuring recombinationlifetimes associated with the transition metal in the entireheat-treated second reference sample; (f) producing a correlation linebetween the concentration of recombination centers and the recombinationlifetimes from measurement results obtained in the step (d) and the step(e), and defining regions including at least the region [Pv] and theregion [Pi] as well as a boundary thereof in the first reference sample;(g) slicing a second silicon single crystal ingot in an axial directionthereof, the second silicon single crystal ingot pulled from a secondsilicon melt at a predetermined pull rate, to prepare a measurementsample including at least a region [Pv] and a region [Pi]; (h) coating asurface of the measurement sample with a third transition metal solutionthat is the same as the second transition metal solution tometal-contaminate the measurement sample; (i) heat-treating themetal-contaminated measurement sample under the same conditions as thosein the step (c) to diffuse the transition metal in the measurementsample; (j) measuring a recombination lifetime associated with thetransition metal in the entire heat-treated measurement sample; and (k)checking results of the measuring in the step (j) against thecorrelation line to infer the region [Pv] and the region [Pi] as well asa boundary thereof; wherein the first and second reference samples andthe measurement sample that have been sliced are such that an oxygenconcentration thereof is within the range of 8.0×10¹⁷ to 1.0×10¹⁸atoms/cm³, or that the boundary between the region [Pv] and the region[Pi] is unidentifiable in the samples when their recombination lifetimesare measured after the samples are heat-treated at 800° C. for 4 hoursin a nitrogen atmosphere and subsequently further heat-treated at 1000°C. for 16 hours; and wherein the region [V] is a region in whichvacancy-type point defects are predominant and defects in whichexcessive vacancies are agglomerated are contained, the region [Pv] is aregion in which vacancy-type point defects are predominant and defectsin which vacancies are agglomerated are not contained, the region [Pi]is a region in which interstitial silicon-type point defects arepredominant and defects in which interstitial silicons are agglomeratedare not contained, and the region [I] is a region in which interstitialsilicon-type point defects are predominant and defects in whichinterstitial silicons are agglomerated are contained.
 6. The methodaccording to claim 5, wherein the transition metal is Fe, Ni, Cu, or Co.7. The method according to claim 6, wherein, when the transition metalis Ni or Co, a second reference sample and the measurement sample arechemically etched at a thickness within the range of 500 to 1000 μmafter the second reference sample and the measurement sample have beencoated with the solution in which Ni or Co is dissolved and have beenheat-treated, and their recombination lifetimes associated with thetransition metal are measured before and after the chemical etching ofthe second reference sample and the measurement sample.
 8. The methodaccording to claim 6, wherein the first reference sample is coated witha solution in which Fe is dissolved, and the second reference sample andthe measurement sample are coated with a solution in which Ni or Co isdissolved.
 9. The method according to claim 5, wherein the concentrationof recombination centers formed in the silicon single crystal by theheat-treating for diffusing the transition metal is measured using aDLTS (deep level transient spectroscopy) method.
 10. The methodaccording to claim 5, wherein the recombination lifetimes subsequent tothe heat-treating for diffusing the transition metal is measured usingan LM-PCD (laser/microwave photoconductivity decay) method.
 11. A methodof identifying a point defect distribution in a silicon single crystalingot, comprising the steps of: (a) slicing a silicon single crystalingot in an axial direction thereof, the ingot pulled from a siliconmelt at a varying pull rate, to prepare first and second samples eachincluding a region [V], a region [Pv], a region [Pi], and a region [I];(b) measuring oxygen concentrations of the first and second samples; (c)subjecting the first sample to a first heat treatment at 800° C. for 4hours in a nitrogen or oxidizing atmosphere and subsequently to a secondheat treatment at 1000° C. for 16 hours, when the oxygen concentrationsof the first and second samples are 1.2×10¹⁸ atoms/cm³ or higher; (d)measuring recombination lifetimes in the entire heat-treated firstsample; (e) defining a boundary between the region [Pi] and the region[I] in the first sample based on measurement results in the step (d);(f) subjecting the second sample to a third heat treatment at 1100 to1200° C. for 1 to 4 hours in an oxidizing atmosphere; (g) selectivelyetching the second sample subjected to the third heat treatment; (h)observing the selectively-etched second sample with an opticalmicroscope to identify an oxidation induced stacking fault (OISF)region; and (i) defining a boundary between the region [V] and theregion [Pv] in the second sample based on a result of the observing inthe step (h); wherein the region [V] is a region in which vacancy-typepoint defects are predominant and defects in which excessive vacanciesare agglomerated are contained, the region [Pv] is a region in whichvacancy-type point defects are predominant and defects in whichvacancies are agglomerated are not contained, the region [Pi] is aregion in which interstitial silicon-type point defects are predominantand defects in which interstitial silicons are agglomerated are notcontained, and the region [I] is a region in which interstitialsilicon-type point defects are predominant and defects in whichinterstitial silicons are agglomerated are contained.
 12. A method ofidentifying a point defect distribution in a silicon single crystalingot, comprising the steps of: (a) slicing a silicon single crystalingot in an axial direction thereof, the ingot pulled from a siliconmelt at a varying pull rate, to prepare first and second samples eachincluding a region [V], a region [Pv], a region [Pi], and a region [I];(b) measuring oxygen concentrations of the first and second samples; (c)subjecting the first sample to a third heat treatment at 1100 to 1200°C. for 1 to 4 hours in an oxidizing atmosphere when the oxygenconcentrations of the first and second samples are 9.0×10¹⁷ atoms/cm³ orlower; (d) selectively etching the first sample that has been subjectedto the third heat treatment; (e) observing the selectively-etched firstsample with an optical microscope to identify an oxidation inducedstacking fault (OISF) region; (f) defining a boundary between the region[V] and the region [Pv] in the first sample based on a result of theobserving in the step (e); (g) selectively etching the second sample;(h) observing the selectively-etched second sample with an opticalmicroscope to identify an interstitial-type large dislocation region;and (i) defining a boundary between the region [Pi] and the region [I]in the second sample based on a result of the observing in the step (h);wherein the region [V] is a region in which vacancy-type point defectsare predominant and defects in which excessive vacancies areagglomerated are contained, the region [Pv] is a region in whichvacancy-type point defects are predominant and defects in whichvacancies are agglomerated are not contained, the region [Pi] is aregion in which interstitial silicon-type point defects are predominantand defects in which interstitial silicons are agglomerated are notcontained, and the region [I] is a region in which interstitialsilicon-type point defects are predominant and defects in whichinterstitial silicons are agglomerated are contained.
 13. A method ofidentifying a point defect distribution in a silicon single crystalingot, comprising the steps of: (a) slicing a silicon single crystalingot in an axial direction thereof, the ingot pulled from a siliconmelt at a varying pull rate, to prepare first and second samples eachincluding a region [V], a region [Pv], a region [Pi], and a region [I];(b) measuring oxygen concentrations of the first and second samples; (c)subjecting the first sample to a first heat treatment at 800° C. for 4hours in a nitrogen or oxidizing atmosphere and subsequently to a secondheat treatment at 1000° C. for 16 hours, when the oxygen concentrationsof the first and second samples are 9.0×10¹⁷ atoms/cm³ or less; (d)measuring recombination lifetimes in the entire heat-treated firstsample; (e) defining a boundary between the region [Pi] and the region[I] and a boundary between the region [V] and the region [Pv] in thefirst sample based on measurement results in the step (d); (f)subjecting the second sample to a fourth heat treatment at 700° C. tonot more than 800° C. for 4 to 20 hours or at 800° C. for more than 4 to20 hours in a nitrogen or oxidizing atmosphere and subsequently to afifth heat treatment at 1000° C. for 1 to 20 hours; (g) measuringrecombination lifetimes in the entire heat-treated second sample; and(h) defining a boundary between the region [Pi] and the region [I] and aboundary between the region [V] and the region [Pv] in the second samplebased on measurement results in the step (g); wherein the region [V] isa region in which vacancy-type point defects are predominant and defectsin which excessive vacancies are agglomerated are contained, the region[Pv] is a region in which vacancy-type point defects are predominant anddefects in which vacancies are agglomerated are not contained, theregion [Pi] is a region in which interstitial silicon-type point defectsare predominant and defects in which interstitial silicons areagglomerated are not contained, and the region [I] is a region in whichinterstitial silicon-type point defects are predominant and defects inwhich interstitial silicons are agglomerated are contained.